Missing type detector

ABSTRACT

A moving train of type elements associated with a high speed printer are passed through the gap of an inductor. The inductor, which is part of a tank circuit of an oscillator, provides a magnetic field across the gap which induces eddy currents in the type elements. The oscillator provides an output signal which is responsive to the changes in the effective resistance of the inductor caused by changes in the eddy currents whenever at least a predetermined portion of at least one of the type elements is missing.

United States Patent 1 Buckley et a].

[541 MISSIVNG TYPE DETECTOR [75] Inventors: Frederick Buckley, Vestal;George Melnyk; Bruce A. Wolfe, both of Endicott, all of NY.

73 Assignee: International Business Machines 7 Corporation, Armonk, NY.

22 Filed: Feb. 24, 1972 21 App1.No.: 228,979

[52] US. Cl ..307/149, 317/D1G. 2, 331/65 3,314,006 Hentschel ..324/41 1Apr. 10, 1973 3,344,346 9/ 1967 Halsey ..324/40 3 ,4 1 6,071 12/ 1 968Wiers ..324/40 3,449,661 6/1969 Puidak ..324/34 3,453,532 7/1969Gardiner ..324/3 3,469,204 9/ 1969 Magyar et al.... 1 7/D1G. 2 3,489,9551/1970 Freebom ..317/123 3,521,184 7/1970 Bowker ..331/65 3,601,6918/1971 Gardiner ..324/43 3,603,874 9/1971 Posey ..324/40 3,605,6109/1971 McDowell et a1. ..101/93 3,626,344 12/1971 Shatemikov et a1..'.....336/13 Primary Examiner-L. T. Nix

Att0meyNorman R. Bardales et a1.

7 [57] ABSTRACT A moving train of type elements associated with a highspeed printer are passed through the gap of an inductor. The inductor,which is part of a tank circuit of an oscillator, provides a magneticfield across the gap which induces eddy currents in the type elements.The oscillator provides an output signal which is responsive to thechanges in the effective resistance of the inductor caused by changes inthe eddy currents whenever at least a predetermined portion of at leastone of the type elements is missing.

7 Clains, 4 Drawing Figures IPATENTEUAFRIOISYS 27.075

SHEET 1 UF 2 -0UTPUT FIG. 4'

. 1 I MISSING TYPE DETECTOR ROSS-REFERENCE F RELATED APPLICATIONS Thisinvention is particularly useful for high speed printers such as the onedescribed in co-pending patent application, Ser. No. 884,953, filed Dec.15, 1969, en-

titled High Speed Front Printer, James M. Cunningham and assigned to thepresent assignee herein,

and now 'us. Pat. No. 3,'653,32l, patented Apr. 4,

1942 The aforementioned co-pending application is incorporated byreference herein.

BACKGROUND OE PIE INVENTION I 1. Field of the Invention v r Thisinvention is'related to missin type detectors for I in the effectiveresistance component of the inductor is bodim'ents of the invention, asillustrated in the accomprinters and is particularly useful for highspeed prin- Y ters. 2. Description of the Prior Art If in a printer amissing type is undetected, a gap in the printout results. Heretoforeinthe low speed printers of the prior art .which employedmovable type, de-

inducing eddy currents in the moving type elements.

The output signal means is responsive to changes in the effectiveresistance of the inductor means caused by the changes in the eddycurrents induced in the type elements whenever at least a predeterminedportion of at least one of the type elements is missing As contemplatedby the present invention,a change utilized rather than a change of itsreactive component, thereby simplifying the attendant problemsassociated with the requirements for sensing a change in the reactivecomponent.

The foregoing and other objects, features and advantages of theinvention will be apparent from thefollowing more particular descriptionof preferred empanying drawing.

BRIEF DESCRIPTION OF THE DRAWING tection of a missing type wasaccomplished through visual observance by "the operator. This prior arttechnique for detecting missing type was not very reliable orsatisfactory since it was subjectto human error.

' With the advent of high speed printers,.the detection of missing typebecame even more acute. For example, a broken piece of type,if notdetected, besides causing a gap in the printout, could cause seriousphysical damage to the printer apparatus and/or operator especially ifit were to be propelled by coming into contact with some high speedmoving part of the printer. Thus, the reliance upon a human observer todetect for a missingtypewas 'notonly unreliable but impractical in thecase of high speed printers. I

The use of an automatic detector system is thus dictated. While anautomatic optical detector system is suggested, it hascertaindisadvantages. For example,

the optical detectors are subjectto external andhence erroneousstimulation suchas light leakage, or

degradation as might be'the case of dirt and dust type particles-fromthe paper being printed on and generally associated'with the printerenvironment. Thus, automatic optical detectors are also not deemedpractical for thispurpose. I I

SUMMARY OF THE INVENTION 'It is 'an object of this invention to providea simple and automatic missing type detector circuit apparatus for amovable typeprinter andswhich is particularly useful for high speedprinters.

Another object of the. invention is to provide the,

aforementioned detector circuit apparatus which isimpervious'to-external stimulation caused by dirt, dust, electricalnoise, stray magnetic fields and the like.

According to oneaspect of the invention, there is provided in a movingtype printer having a train of type FIG. 1 is a schematic diagram inpartial block form of an embodiment of the circuit apparatus of thepresent DESCRIPTION OF THE PREFERRED EMBODIMENTS For sake ofexplanation, it will be assumed that the missing type detector circuitapparatus of the inventive embodiments described'herein are part of'highspeed front printers such as the one described in the aforementionedco-pending application, Ser. No. 884,953.

elementsfmoving in a predeterminedrpath, a missing tector circuitapparatus has oscillator circuit-means and output signal means. Theoscillator circuit means has tank circuit means which has inductor meanswith a gap through which the moving type elements pass. The

. inductor means provides a magnetic field in the gap for type detectorcircuit apparatus. The missing type de- Briefly, in the high speed frontprinter described in the last-mentioned application, a plurality ortrain of type carriers are guided along a print line by a continuousguide" rail. Each typecarrier has a plurality of pivotal levers carryingraised type characters adjacent their free ends. The type carrier leversare impacted by selected type hammers in the different print positionsto impact a document and a ribbon or the like for printing in selectedpositions as the type carriers move by the print positions. For a moredetailed description of the apparatus andoperation of this high speedfront printer, reference can be made to the aforementioned co-pendingapplication, Ser. No. 884,953, which as aforementioned is incorporatedby reference herein.

Referring now to FIG; l,-for sake of clarity only a part of one typeelement is shown. More specifically,

'the part ofthe type element which is illustrated in'partially brokenaway form in FIG. 1 is the raised type character 11b and cam surfacellr, which are described and referred to in the aforesaid co-pendingapplication and designated therein with the same referencecharacteristics band [In It should be. understood, that there is aplurality or train of such type elements and that in operation the typeelements move in a path which is normal to the plane of FIG. 1. I

In accordance with the. principles of the present invention, the trainof moving type elements are passed through the gap 1 of a suitableinductor 2. Inductor 2 provides an alternating magnetic field in gap 1.

. Preferably, inductor 2 is of the type comprising a ferrite core 3 withopposing pole faces 4, 5, which define gap 1. It also includes a pair ofwindings 6, 7, which are wound on core 3 in a series aidingrelationship.

If all the type elements are present, 'a certain level of eddy currentsare induced in the moving type elements which tend to counteract oroppose the changes in the magnetic field in gap 1 than would ordinarilybe the case if gap 1 was empty. Accordingly, as some predeterminedportion of one or more of the type elements becomes missing the magneticfield strengthin the gap increases due to the decrease in the eddy cur-Node 24 is connected to a suitable signal detector circuit 25.,Preferably, the output signal at node24 is fed to the input of apositive peak follower, which in the embodiment of FIG. 2 is generallyshown as the network of amplifier 26, clipping diode 27, capacitor 28and resistor 36. Amplifier 26 is preferably configured as a differentialamplifier and its noninverted input is connected to node 24. The peakfollower extracts the positive sinusoidal envelope of the oscillatoroutput signal and provides an analog voltage V1 atnode 35 which isproportional to the resistance 8. The voltage V1 is fed back to theinverted input of amplifier 26, as

well as to an input of a comparator 29. Comparator 29 as configured inFIG. 1- also is a differential amplifier and its inverted input isconnected to node 30 and hence node 35. The non-inverted input ofcomparator rent level resulting from the diminution of the physical 29is connected to node 31. The output of comparator 29 is taken atterminal 29A across grounded resistor More particularly, the latter iseffected by combining the inductor '2 with condenser 9 to form a tankcircuit 10. The tank circuit 10 in turn is preferably part of thepositive feedback path of an oscillator 20. and determines theoscillators amplitude and frequency of oscillation. Oscillator 20 whichis generally indicated by the legend OSCILLATOR in FIG. 1, includes acurrent amplifier l2 and gain and limiting amplifier 13. Currentamplifier 12, which is configured as an NPN transistorized type, drivestank circuit 10. Its collector is connected to one end of the tankcircuit 10. Its emitter is connected to a bias resistor 14. Suitablepower supply means, not shown, are connected to the terminals 15 and 16.

Amplifier 13 is preferably configured'as a differential amplifier. Itsnoninverted input is grounded as shown in FIG. 1. The loop gain of theoscillator is limited by the parallel connected back-to-back diodes 17,18, connected across the output of amplifier 13 and its inverted input.The output of amplifier 13 is also connected to the control or baseelectrode of current amplifier 12. The loop gain'control is such thatthe signal current driving tank circuit 10 is substantially constant inamplitude and is periodically provided at the tank circuit' s resonancefrequency. Consequently, the amplitude of the sinusoidal voltage acrosstank cireuit10 is proportional to the effective resistance 8. It ispreferable to provide a buffer amplifier 19 in the feedback path of theoscillator to minimize loading of the tank circuit 10.'In the embodimentof FIG. 1, amplifier 19 is configured as a differential amplifier withan appropriate feedback loop shown schematically as a conductor 19aconnected across its output to its inverting input. The output signal-ofamplifier 19 is a.c. coupled via capacitor 21 and resistor 22 to theinverting input of amplifier 13. At resonance, the loop gain for smallsignals is approximately the product of resistors 8 and 23 divided bythe product of resistors 14 and 22. An output is taken from theoscillator at node 24.

298. Also connected to node 31 is a constant current source, not shown,which provides a reference voltage V2-V1 across nodes 31 and 35. In thisregard, the RC integrator 32-33, in connection with the current source,not shown, provides a reference voltage V2 at node 31, which is comparedwith the voltage V1 by comparator 29. A suitable reference supply, notshown, is connected to terminal 34 which is connected to node 35 viaresistor 36.

The operation of the circuit of .FIG. 1 will now be described. For theabove-described loop gain condition,-'i.e. a small signal condition, asthe voltage at the output or node of the oscillator increases, it causesa corresponding increase-in the current passing through resistor 22. Theincreasing current, which is fed to the inverting input of amplifier 13,thereby decreases the output voltage of amplifier 13. In turn, thisvoltage.

decrease at the output of amplifier 13, which is applied to the base oftransistorized amplifier l2, causes a decrease in the emitter voltage ofamplifier 12. As a result, the collector current of amplifier 12, whichcurrent drives tank circuit 10, decreases and thereby causes a reductionof the voltage across the tank circuit 10 resulting in the voltageincreasing at node 24. The

now increasing voltage at node 24 causes a corresponding decrease in thecurrent passing through resistor 22 thereby increasing the outputvoltage of amplifier 13. As a result, the collector current of amplifier12 increases and the output voltage of the circuit 10 and resultingvoltage at node 24 decrease and the cycle repeats.

The collector current which drives the tank circuit 10 causes the latterto ring or oscillate in a manner well known to those skilled in the artat its resonant frequency and as a consequence, the voltage across thetank, and hence at node 24, is substantially sinusoidal. However, whilethe collector current is varying periodically, its effective value issubstantially constant. Thus, the amplitude of the sinusoidal voltageacross tank circuit 10 is proportional to the effective resistance 8.Consequently, any increase in the effective resistance 8, which iscaused by one or more missing type elements or predetermined portionthereof and concomitant reduction of eddy currents induced in theremaining type elements, will cause an increase in the IR drop acrossresistance 8 and hence in the respective amplitudes of the outputsignals of tank circuit 10 and at node 24. As is obvious to thoseskilled in the art, the sinusoidal voltage at node 24 is converted bythe coaction of amplifier 13 and diodes 17, 18 to a square wave pulsetrain of the frequency of the sinusoidal voltage. The square wavevoltage train, in turn, causes the current source, i.e. transistor 12,to change at the same frequency between two conduction levels.

The sinusoidal output at node 24 is fed to the aforementioned peakfollower network of detector 25 which provides the extracted envelopesignal V1 at node 35. Referring to FIG. 2, for sake of explanation itwill be assumed that prior to time t0 all the type elements of the typetrain are present and passing through gap 1, and that thereafter everyother type element in the train just prior to passing through gap 1 ismissing, e.g. becomes broken off.

Accordingly, just prior to time t0, the voltage V1 will be at aquiescent level E1, c.f. axis A of FIG. 2, which is associated with thecondition of all type elements being present and passing through gap 1.More particularly under this condition, the eddy currents induced in thetype elements are at some maximum level and the inversely proportionalresistance 8 is at ,some corresponding low value. The amplitudes of theresultant sinusoidal IR drop across tank circuit and sinusoidal outputat node 24 are hence at their correspondingly lowest values. Thus, whenthe peak follower extracts the envelope wave shape of the sinusoidalsignal appearing at node 24, its output signal ,Vl will be at the lowerlevel E1 shown in FIG. 2. Under this same condition, the constantcurrent source, not shown, connected to node 31 provides an IR dropacross resistor 32 which maintains the voltage V2 present at node 31 atsome higher quiescent level E2, the drop AV across resistor 32 beingsubstantially constant. As a result, the output signal Eout, c.f. axis Bof FIG. 2, at output 29A of comparator 29 is at a null, e.g. level Exshown in FIG. 2.

' At t0, the portion of the type train with thefirst missing typeelement begins to pass through gap 1. For sake of clarity, the firstmissing type element of the train is indicated by the dash line 110,c.f. axis C. It should be understood that the vertical dash linesassociated with the waveform of axisC represent the alternate missingtype elements, the vertical solid lines represent the remaining typeelements, and the arrow D indicates their direction of their movement.The void in the train caused by the missing type element causes the eddycurrent level induced in the remaining type elements to decrease. Thisdecrease results in the effective resistance 8 increasing and theamplitudes of the output signals across the tank circuit 10 and node 24to increase. As a result, signal V1 at node 35 begins to rise at time t0due to the increase in amplitude of the signal at node 24. Signal V2also begins to rise at time t0 but at a slower rate than V1, thecapacitance of capacitor 33 being judiciously selected to be larger thanthat of capacitor 28 for this purpose.

At t1, signal V1 crosses over signal V2, i.e. V1

becomes larger than V2. Signal Eout in response to the crossover goesfrom its null level Ex to a different level Ey, which change isindicative of the presence of a missing type condition. In practice andpreferably, when such a condition occurs the change in signal levels ofsignal Eout would be fed to a control system,

not shown, which stops the movement of the type train and/or othermoving parts of the printer. For example, the output 29A may be coupledto a latch circuit which is triggered and set to its latched state bythe aforesaid change in the signal level of signal Eout at time :1.

However, for sake of explanation, it will be assumed that the type traindoes not stop at time t1. Accordingly, the change in the eddy currentlevel induced in the remaining type causes signal V1 to reach some levelEPl whereupon the signal V1 attempts to return to its quiescent level Elas the void created by the missing type element becomes more and moreremoved from gap 1. At time :2, signal V1 once again goes below signalV2, and the output signal Eout returns to its null level Ex. If no othertype were missing, the signals V1 and V2 would return to theirrespective quiescent levels E1 and E2, and the cycle would not repeatuntil the next time, e.g. time t6, when the void created by theparticular missing type elements again commences to pass through gap 1.

For the assumed condition of alternately missing type elements andnonstoppage of the type train, at time :3 the void created by the nextmissing type element 112 in the train commences to pass through gap 1.Asa result, signals V1 and V2 begin to rise and at time t4 signal V1crosses over signal V2. Signal Eout of comparator 29 again switches fromlevel Ex to level Ey. After the signal V1 reaches its peak EP2, it againproceeds to descend toward its quiescent level E1. When signal V1 goesbelow signal V2 at time t4, signal Eout reverts back to its null levelEx.

Each successive missing type void continues the cycle. The signal levelsE3 and E4 represent the levels of signals V1 and V2, respectively, if notype elements are in gap 1 such as might be the case when the type trainis removed for some purpose, e.g. maintenance, replacement, cleaning,etc. Consequently, for the assumed case where all the type elementsbecome alternately missing just prior to passing through gap 1 and thetype train continues to move, then eventually at time t5 when somesubsequential part of or all of the type voids have gone through gap 1signals V1 and V2 will become asymptotic to the respective levels E3 andE4 and the output level of comparator 29 would remain at Ex. In theassumed condition, it should be noted that the pulse widths of signalEout become progressively smaller.

It should be understood that if the spacing between the voids formed bythe missing type elements in the train is large enough, the signals V1and V2 would return to their respective quiescent levels E1 and E2between peaks. In general, as can be readily appreciated by thosefamiliar with the art, the waveform characteristics shown in FIG. 2 willbe a function of the number of and spacing between type elements, thelength and speed of the train, the number of and spacing between voids,the dimension of the void(s), and the time constants associated with theintegrator 32, 33 and peak follower.

As previously mentioned, however, as contemplated by the preferred modeof operation, the type train and/or other moving parts of the printerwould be stopped as soon as the first change in the signal level ofsignal Eout occurs.

.not shown, passed through the gap 41 of an inductor 42. Inductor 42 hasa generally C-shaped core 43 with pole faces 44, 45 that define the gap41. A pair of windings 46, 47 are wound on the legs of core 43 in aseries aiding relationship. The effective resistance 48of inductor 42 isshown in dash line form for sake of clarity. Inductor 42 forms aparallel tank circuit with the grounded capacitor 49. In this regard,windings 46,47 of inductor 42 are ac. coupled to ground via a.c.coupling capacitor 50.. 7

. An NPN current source amplifier 12' drives the tank circuit 42-49which is connected to its collector. The emitter of amplifier 12" isconnected to' bias resistor '14.

A pair of NPN transistors 51, 52 are configured as a differential pairin the input stage of the gain and limiting amplifier generallyindicated by the reference numeral 13'. As such,-the respective bases oftransistors 51 and' 52 act as the noninverting and inverting inputs,respectively, of the differentialamplifier. The emitters of transistors51, 52 are commonly connected to a bias.- ing network shown as seriesconnected diode 53 and resistor 54. A' negative power supply, not shown,is connected tothe terminal 56 and provides an appropriate negative,voltage Vy. The last-mentioned power supply provides via conductor 57an appropriate bias voltage to other points in the circuitof FIG. 3, ashereinafter described. De-coupling capacitor 58"provides a.c.signalde-coupling to ground at node 59. In a particular hybridcircuitmodule used to implement the differential amplifier of oscillator40, diode 53 was present. It should be understood, however, that the useof diode 53 is optional and may be obviated if the differential.amplifier is implemented by other circuit types. Y

, A positive power-supply, not shown, is connected to the terminal 60'and provides a positive voltage Vx. It provides a bias voltage at node62 which is distributed to various points in the circuit of FIG. 3, ashereinafter described. A diode voltage divider network of three seriesconnected diodes 63, 64, and 65 are connected across the nodes 62 and66. Network 63 -65.in conjunction with the resistor'67 and groundeddiode 70 provides a voltage at node 68 which is applied to the base oftransistor 69. The base of transistor 51 is grounded. The-emitter oftransistor 69 is connected to the collector of transistor 52. Thecollector of transistor 51 is biased by the voltage at node 62, whereasthe collector of transistor 69 is connected to the node 62 via asuitable bias resistor 71. The collector output of transistor 69 isconnected to the base of transistor 72 which is configured as an emitterfollower. Series connected resistor 73 and capacitor 74, which areconnected between node 62 and the base input of transistor 72, frequencystabilize amplifier 13 in a manner familiar to those skilled in the art.Transistor 72 has its collector connected to node 62.

Transistor 69 provides improved response for amplifier 13'. Transistor72 in coaction with diode 75 bptimizes the operation of transistor 12 inthe linear range. The emitter of NPN transistor'72 is connected viadiode and node 75A to parallelconnected backto-back diodes 76, 77.Resistor 75B acts as a bias for transistor 72. The diodes 76, 77 areconnected to the inverted input of the differential amplifier pair 51,52 via resistor 78. A feedback resistor 79 is connected in parallel withthe feedback diodes 76, 77. Diodes 76 and 77 limit the loop gain ofoscillator 40 in a manner similar to the way diodes 17 and 18 limit thegain of oscillator 20 of FIG. 1.

In the embodiment of FIG. 3, the buffer amplifier 19' provided in thefeedback path of oscillator 40 is configured as'an NPN emitter follower.As such, amplifier 19j has the base of its transistor 80 connected tothe node 81 of tank circuit 42-49. The collector of transistor 80 isconnected to node 62.

An output is taken from oscillator 40 at node 82 which connects thejunction of the emitter of transistor 80 and the bias resistor 83. Theoutput signal at node 82 is fed back to the inverted input of transistor52 via resistor 22', capacitor 21 and resistor 78.

In the embodiment of FIG. 3, the peak follower stage i 26 is configuredas an emitter follower. As such, the

base of NPN transistor 86 is connected to node 82.. Its output isconnectedto an input of comparator 29' and to the node 35 It providesthe extracted envelope signal VI. The collector of transistor 86 isconnected to node 62. Its emitter is bias connected to node 59 via anappropriate bias resistor 87 connected in the emitter circuit for thispurpose.

Comparator 29 includes NPN transistors 88 and 89 which are configured asa differential pair. The base of transistor 88 is the inverted input andthe base of transistor 89 is the noninverted input. The emitters oftransistor 88, 89 are commonly connected to a current source 90 viadiode 91. Current source 90 is an NPN transistor and has transistors 88,89 in its collector circuit; The emitter of transistor 90 is connectedto the node 59 via resistor 92. The base of transistor 90 is biasedacross node 62 and node 59 via the biasing network which includesresistor 93 and series connected diodes 94, 95. PNP transistor 96 isconnected to the collector of transistor 88. The emitter of transistor96 is connected to node 62 and its base is connected via resistor 97 tothe same node. The collector of transistor 96 isconnected to the voltagedividing network resistors 98, 99 to node 59. Transistor 96 in coactionwith resistors 98, 99 provides appropriate control levels for switchingoutput transistor 100 to ON or OFF conditions. Comparator 29 includes inits output an NPN emitter grounded transistor switch 100. The baseelectrode'of transistor 100 is connected to the junction of resistors98, 99. Output of comparator 29' istaken from the collector oftransistor 100.

Node 35 is connected to the grounded capacitor28',

which 'is part of the peak detector circuit 26'. Resistor 32' isconnected across the nodes 35' and 31 and together with the capacitor 33forms an RC integrating circuit. Current source 101 is connected to thenode 31' and provides a reference voltage V2-V1' across nodes 31' and35'. Source 101 is configured as a PNP transistor 102, the collectoroutput of which is connected to the aforesaid node 31'. The emitter oftransistor'l02 is connected via resistor 103 to the node '62 and itsbase is connected via conductor 50 to nod 104 in diode network 63-65.

The principles of operation of the embodiment of FIG. 3 is similar tothe embodiment of FIG. 1, and

hence for sake of brevity are omitted.

' cuit apparatus of FIG. 3 are indicated in Table I, as follows:

TABLE I Transistors: NPNs l2,69,72.80,

86, 90, I Fairchild Type FTI 3 12, each NPNs51,52,88,

89 Texas :lnstrument Type 2N2639 I eac PNPs 96, 102 Texas InstrumentType 2N241 1 each DIODES 53,63, 64,65,

70,75,76,77, 91, 94, 95 Fairchild Type FD7, each Resistors 14', 22', 54,75B, 83 1,000 ohms, each Resistors 32', 98 2,000 ohms, each Resistor 67,73 510 ohms,'each Resistor 71 560 ohms Resistor 78 1,800 ohms Resistor79 6,200 ohms Resistor 87 22,000 ohms Resistor 92 390 ohms Resistor 933,900 ohms Resistor 97 750 ohms Resistor 99 10,000 ohms Resistor 1035,100 ohms Capacitor 21 0.1 pf Genes i .0515 Capacitors 3 3 50,

58 6.8 uf,each Capacitor 49 910 pf Capacitor 74. 30 pf Vx +6 volts d.c.Vy -3 volts d.c.

For the component values of Table I, a typical tank circuit 42-49 has atuned frequency range of approximately 1.629 megahertz to 1.608megahertz corresponding to all the'type elements being present andpassing through the inductor gap and to zero or no type elements,respectively. The bandwidth was approximately 0.200 megahertz. Typicalparameters of the inductor 42 are as follows: twenty turns for eachwinding 46, 47 which may be of No. 38 AWG or No. 40 AWG, for example; agap spacing between pole faces 44 and 45 of 0.125 inches; a corecomposition of a ferrite material with a low resistivity at the desiredresonant frequency; and a core cross sectional area of 0.00375 squareinches.

By judiciously selecting a narrow bandwidth for the oscillator frequencyand/or the geometry of the core, the system of the invention isrelatively imperious to external stimulation caused by stray magneticfields.

Referring to FIG. 4, there is shown the frequency response of a typicaltank circuit for the conditions of (I) all type elements present, (II)one type element missing, and (III) no type elements present relative tothe inductor gap and under static conditions, i.e. the type elements arestationary. Referring to waveform I, with all the type elements present,i.e. aforementioned condition (I),the tank impedance measured was 0.96kilohms at a tuned tank frequency of 1.629 megahertz. For a fixed tankcapacitance of 1,000 picafarads, the computed tank inductance was 9.55microhenries. As

such, i.e. under tuned conditions, the real part of the tank impedance,which is substantially the effective parallel resistance of the tankinductor, is approximately the 0.96 kilohms.

For the same tank capacitance, where one type element was missing andthe void created thereby was present in the inductor gap, the tankimpedance measured 2.72 kilohms at a tuned tank frequency of 1.608megahertz, c.f. waveform (II). Under this condition, i.e. condition(11), the inductance calculated was 9.8 microheriries, and the effectiveparallel resistance of the tank inductor was approximately the 2.72kilohms.

Again for the same tank capacitance, and for condition (III) with allthe type elements removed, the tank impedance and hence effectiveparallel resistance of the tank inductor measured 4.4 kilohms at a tunedtank Thus, the waveforms I, II, III indicate that the effectiveresistance of the inductor increases as the amount of missing typeelements increases, whereas the inductance and resonant frequency remainsubstantially constant. As the number of missing type elementsincreases, the eddy currents induced in the remaining type elementsdecrease as previously explained. Accordingly, the effective resistanceof the inductor is inversely proportional to the change in eddy currentin the remaining type elements.

One possible theory of explanation for the change in the effectiveresistance of the inductor vis-a-vis the change in the induced eddycurrent in the remaining type elements can be explained by the followingdescribed mathematical relationships.

More specifically, the publication The Electromagnetic Field In ItsEngineering Aspects, G. W. Carter, 2nd Edition, 1967, American ElsevierPublishing Company, Inc., page 247 gives the magnetic flux densityphasor relationship B of a block material located in an alternatingmagnetic field as follows:

B flux density at the surface of the block.

at the distance along the x axis of an x-y coordinate system, the originof which system is located at the surface of the block, and

d is the effective depth of which the material of the block ispenetrated by the flux of the magnetic field.

Furthermore, at page 245 of the aforesaid publication, the effectivedepth d is given as follows:

d= V p/ucu where: v

p is the resistivity of the block,

to is the frequency of the alternating field, and

p. is the permeability of the block.

The flux linkage N associated with an inductor creating the flux densityB is as follows:

A cross section of the inductor core,

N the number of turns of the inductor winding,

L the inductance of the inductor without any material in the gap, and

I current in the inductor.

voltage v across it is as follows:

".Substitutingand transposing equations (1), (3) and Z ff=v/I j L-i(1+hm VT 7 where Zeff is the effective impedance of the indu tTransposing the current term I in equation (5).results with the block ofmaterial in the gap. The impedance .Zeff can beexpressed as, a parallelLR circuit with the following relationships:

Leff= Le' V Cos x/d W Rerr=Lfl- V Sin x/d" VT, Accordingly, for theconditionof an approximately constant w,increasing p., and decreasing p,such as is the case of material in the gap, then d decreases, the sineterm of equation (8) increases, the exponential term decreases and Reffdecreases. On the other hand,

for the same condition, the cosine term of equation (7) decreases whilethe exponential term is decreasing so that the net changein Leff isnegligible. As more-and more material is missing in the gap, Reffincreases but changes in Lerf still remain negligible.

It should be understood thatthe type elements are made of a conductivematerial suitable for having eddy wherein said output signal meansfurther comprises:

comparator means having first and .second input means and first outputmeans for providing said output signal, peak detector means having thirdinput means and second output means, said second output means v whereinsaid oscillator circuit means further comprises:

currents induced into it, e.g. steel or the like.

Moreover, while the embodiments have been described with certain circuitimplementations, configurations,

j and the like, or with. transistors of certain and mixedconductivity-types, it should be readily understood that the inventionmay be practiced with other circuit implementations, configurations,and/or, with ap- 'propriate changes in the voltage polarities,transistors I of opposite and/or the sameconductivity types as isobvious to those skilled in the art.

an amplifier having input and output means associated therewith, and

positive feedback means coupled between said input and output means,said positive feedback means comprising said tank circuit means. 4.Detector circuit apparatus according to claim 3 wherein said feedbackmeans further comprises:

I a current amplifierfor driving said tank circuit means, said currentamplifier having control input means coupled to said output means,

a feedback impedance network, and coupling means for coupling saidfeedback impedance network between said input means and saidoutput-means. a v

i 5. Detector circuit apparatus according to claim 4 Thus, while theinvention has been particularly shown and described with reference topreferred embodiments, it will be understood by those skilled in the 1art thatthe foregoing and other changes inform and detail may be madetherein without spirit and scope of the invention. Weftclaim: i a j 1.In a moving type printer having a train of typeelements moving in apredetermined path, missing type detector circuit apparatus comprising:

. oscillator circuit means having tank circuit means, and inductor meansincluded in said tank circuit means, said inductor means having a gapthrough which said train of moving type elements pass, and saidinductormeans providing a magnetic field across said gap for. inductingeddy currents in said moving type elements; and output signal meansproviding an" output signal in wherein said coupling means furthercomprises buffer circuit means coupled between said tank circuit meansand :said feedback impedance network, said output signal means beingcoupled to said buffer circuit v 40' departing from the response tochanges in the effectiveresistance of said inductormeans caused by thechanges in said inducted .eddy currents whenev'erat least "apredetermined portion of at least one of said type elementsis missing. a2. Detector circuit apparatus according to claim-1 means.

j 6. Detector circuit apparatus according to claim '5 wherein saidoutput signalmeans further comprises:

comparatormeans having second and third input means and second outputmeans for providing said outputsignal, t a I peak detector means havingfourth input means and third output means, said third output means being.coupled to said second input means, said fourth input means beingcoupled to said buffer circuit means to effectuate the aforementionedcoupling of said output signal means thereat, and reference signal meanscoupled .to said third input ,means for providing a threshold referencesignal thereat, said comparator means providing said output signal atsaid second output means whenever the peak followers output signal atsaid third output means is greater than the threshold reference signal.'I

7. Detector circuit apparatus according to claim 1 when said oscillatorcircuit means is of the'narrowasst-a

1. In a moving type printer having a train of type elements moving in apredetermined path, missing type detector circuit apparatus comprising:oscillator circuit means having tank circuit means, and inductor meansincluded in said tank circuit means, said inductor means having a gapthrough which said train of moving type elements pass, and said inductormeans providing a magnetic field across said gap for inducting eddycurrents in said moving type elements; and output signal means providingan output signal in response to changes in the effective resistance ofsaid inductor means caused by the changes in said inducted eddy currentswhenever at least a predetermined portion of at least one of said typeelements is missing.
 2. Detector circuit apparatus according to claim 1wherein said output signal means further comprises: comparator meanshaving first and second input means and first output means for providingsaid output signal, peak detector means having third input means andsecond output means, said second output means being coupled to saidfirst input means, means for coupling said third input means of saidpeak detector means to said tank circuit means, and reference signalmeans coupled to said second input means for providing a thresholdreference signal thereat, said comparator means providing said outputsignal at said first output means whenever the peak follower''s outputsignal at said second output means is greater than the thresholdreference signal.
 3. Detector circuit apparatus according to claim 1wherein said oscillator circuit means further comprises: an amplifierhaving input and output means associated therewith, and positivefeedbAck means coupled between said input and output means, saidpositive feedback means comprising said tank circuit means.
 4. Detectorcircuit apparatus according to claim 3 wherein said feedback meansfurther comprises: a current amplifier for driving said tank circuitmeans, said current amplifier having control input means coupled to saidoutput means, a feedback impedance network, and coupling means forcoupling said feedback impedance network between said input means andsaid output means.
 5. Detector circuit apparatus according to claim 4wherein said coupling means further comprises buffer circuit meanscoupled between said tank circuit means and said feedback impedancenetwork, said output signal means being coupled to said buffer circuitmeans.
 6. Detector circuit apparatus according to claim 5 wherein saidoutput signal means further comprises: comparator means having secondand third input means and second output means for providing said outputsignal, peak detector means having fourth input means and third outputmeans, said third output means being coupled to said second input means,said fourth input means being coupled to said buffer circuit means toeffectuate the aforementioned coupling of said output signal meansthereat, and reference signal means coupled to said third input meansfor providing a threshold reference signal thereat, said comparatormeans providing said output signal at said second output means wheneverthe peak follower''s output signal at said third output means is greaterthan the threshold reference signal.
 7. Detector circuit apparatusaccording to claim 1 when said oscillator circuit means is of thenarrow-band type.